The ProSavin treatment uses an inert virus to carry corrective genes directly into the striatum region of the brain that controls movement.
It is designed to convert ordinary nerve cells into factories for making dopamine, the signalling chemical that is lost in Parkinson's patients.
Lack of dopamine activity leads to the common Parkinson's symptoms of tremor, slow movement and rigidity.
The trial tested the safety, tolerability and effectiveness of three different doses of ProSavin in 15 patients aged 48 to 65 with advanced Parkinson's disease who were not responding to conventional treatments.
A standard system of rating motor functions was used, covering speech, tremors, rigidity, finger taps, posture, gait, and slow movement. Lower scores indicated better muscle control and co-ordination.
Significant score improvements were seen after six months and a year in all patients not taking medication.
Reporting their findings in The Lancet medical journal, the researchers led by Professor Stephane Palfi, from Les Hopitaux Universitaires Henri-Mondor in Creteil, France, wrote: "ProSavin was safe and well tolerated in patients with advanced Parkinson's disease. Improvement in motor behaviour was observed in all patients."
They stressed that, while promising, the results at this stage were still limited and should be "interpreted with caution".